The treatment of gases and liquids containing acidic gases such as CO2, H2S, CS2, HCN, COS and sulfur derivatives of C1 to C4 hydrocarbons with amine solutions to remove these acidic gases is well established. The amine usually contacts the acidic gases and the liquids as an aqueous solution containing the amine in an absorber tower with the aqueous amine solution passing in countercurrent to the acidic fluid. In typical cases using common amine sorbents such as monoethanolamine (MEA), diethanolamine (DEA), methyldiethanolamine (MDEA), diisopropylamine (DIPA), or hydroxyethoxyethylamine (DGA). The liquid amine stream contained the sorbed acid gas is typically regenerated by desorption of the sorbed gases in a separate tower with the regenerated amine and the desorbed gases leaving the tower as separate streams. The various gas purification processes which are available are described, for example, in Gas Purification, Fifth Ed., Kohl and Neilsen, Gulf Publishing Company, 1997, ISBN-13: 978-0-88415-220-0.
The treatment of acid gas mixtures containing CO2 and H2S with amine solutions typically results in the simultaneous removal of substantial amounts of both the CO2 and H2S. It is often desirable, however, to treat acid gas mixtures containing both CO2 and H2S so as to remove the H2S selectively from the mixture, thereby minimizing removal of the CO2. Selective removal of H2S results in a relatively high H2S /CO2 ratio in the separated acid gas which simplifies the conversion of H2S to elemental sulfur using the Claus process. Selective H2S removal is applicable to a number of gas treating operations including treatment of hydrocarbon gases from oil sands, coal and shale pyrolysis, refinery gas and natural gas having a low H2S /CO2 ratio and is particularly desirable in the treatment of gases wherein the partial pressure of H2S is relatively low compared to that of CO2 because the capacity of an amine to absorb H2S from the latter type gases is very low. Examples of gases with relatively low partial pressures of H2S include synthetic gases made by coal gasification, sulfur plant tail gas and low-Joule fuel gases encountered in refineries where heavy residual oil is being thermally converted to lower molecular weight liquids and gases.
Although primary and secondary amines such as MEA, DEA, DPA, and DGA absorb both H2S and CO2 gas, they have not proven especially satisfactory for preferential absorption of H2S to the exclusion of CO2 because in aqueous solution, the amines undergo more selective reaction with CO2 to form carbamates. The tertiary amine, MDEA, has been reported to have a high degree of selectivity toward H2S absorption over CO2 (Frazier and Kohl, Ind. and Eng. Chem., 42, 2288 (1950)), but its commercial utility is limited because of its restricted capacity for H2S loading and its limited ability to reduce the CO2 content of the gas. Similarly, diisopropylamine (DIPA) is relatively unique among secondary amino alcohols in that its use has been reported, alone or with a physical solvent such as sulfolane, for selective removal of H2S from gases containing H2S and CO2.
U.K. Patent Publication No. 2,017,524-A (Shell) disclosed that aqueous solutions of dialkylmonoalkanolamines, and particularly N,N-diethyl-monoethanolamine (DEAE), have higher selectivity and capacity for H2S removal at higher loading levels than MDEA solutions. Nevertheless, even DEAE is not very effective for the low H2S loading frequently encountered in the industry. Also, DEAE has a boiling point of 161° C., and as such, it is characterized as being a low-boiling, relatively highly volatile amino alcohol. Such high volatilities under most gas scrubbing conditions result in large material losses with consequent losses in economic advantages.
A number of severely sterically hindered amino ether compounds, notably amino ether alcohols, diamino ethers and alkoxy amino ether alcohols have been developed for the selective removal of H2S in the presence of CO2. U.S. Pat. Nos. 4,405,581; 4,405,583; 4,405,585, 4,471,138 and 4,894,178 and U.S. Patent Publication 2010/0037775 disclose these highly effective hindered amino ethers, their synthesis and use in selective gas separation processes. Specific amino ethers described in these patents include BTEE (bis (tert.-butylamino-ethoxy)-ethane synthesized from tertiary-butylamine and bis-(2-chloroethoxy)-ethane as well as EEETB (ethoxyethoxyethanol-tert-butylamine, synthesized from tert-butylamine and chloroethoxy-ethoxyethanol). U.S. Pat. No. 4,894,178 indicates that a mixture of BTEE and EEETB is particularly effective for the selective separation of H2S from CO2. U.S. 2010/0037775 describes the preparation of alkoxy-substituted etheramines as selective sorbents for separating H2S from CO2 Compared to aqueous MDEA, these severely sterically hindered amines lead to much higher selectivity at high H2S loadings
A significant problem arises with some of these absorbent materials during their transport from the manufacturing site to the location of use in cold climates; this problem arises when the pour point of the material is relatively high, typically at least −20° C. and the climatic conditions at their location of use is below that value or close to it. Such climatic zones include, for example, the North Sea areas of the UK and Norway, Ft. McMurray in Alberta, Canada and Billings, Mont. In zones such as these, there is the danger that the liquid will freeze solid or become unpourable to the extent that it cannot be readily or conveniently transferred or used unless they are thawed, but this takes time and provision needs be made for warmed defrosting and storage facilities, especially on offshore platforms where space may be very limited. It would therefore be desirable to transport the sorbents without having them solidify under the temperatures likely to be encountered during shipping from on location to another.